1. Field of the Invention
The present invention relates to a control circuit for controlling the output voltage of a power conversion apparatus such as an inverter and converter, and particularly to a control circuit to obtain accurate and small-distortion sinusoidal output voltage.
2. Description of the Prior Art
In extensive industrial fields, power conversion apparatus such as inverters and converters are used to produce stable and low-impedance electric power. For example, an article entitled "Inverter Output Voltage Waveform Closed Loop Control Technique" in Intelec Papers, pp. 205-212, 18-21, Oct. 1983, published in Tokyo, discloses a technique of controlling the inverter output voltage. FIG. 1 is a block diagram derived from FIG. 5 of the above article on page 207 and redrawn to contrast with the present invention. In the figure, the system includes an inverter main circuit 1, a reactor 2 and capacitor 3 making in combination an alternating current (AC) power filter, a direct current (DC) power source 4, a load 5, a drive circuit 6 for the inverter main circuit 1, an AC reference voltage generator 7 for producing a sinusoidal reference voltage, an amplifier 8, and a pulse width modulation (PWM) circuit 9 consisting of a comparator 9a and a carrier wave generator 9b.
Next, the operation will be described. A sinusoidal output voltage in compliance with the control output of the PWM circuit 9 is produced across the capacitor 3. The amplifier 8 and PWM circuit 9 control the switching operation of the inverter 1 so that the output voltage is equal to the sinusoidal reference voltage of the AC reference voltage generator 7.
The PWM circuit 9 made up of a triangular carrier wave generator 9b and comparator 9a operates to determine the switching time point of pulse width modulation in accordance with a virtually sinusoidal signal produced by amplification of the voltage difference by the amplifier 8. Since the amplifier 8 has a certain finite gain for the stability of operation, the output voltage of the inverter 1 has a small error with respect to the reference voltage of the AC reference voltage generator 8, and the system operates such that the inverter output voltage follows the reference voltage.
The conventional control circuit for a converter such as an inverter is constructed as described above, and the inverter operates as a very low-impedance voltage source when the inverter is seen from the load. On this account, a short circuit current in the load or an inrush current of a transformer tends to cause the inverter to have an excessive output current, and the protection of the inverter has been a matter of difficulty. In addition, when the inverter is loaded by a device such as a rectifier which produces many harmonics, the voltage distortion caused by the harmonics of the load cannot be eliminated completely due to the foregoing principle of control operation in which the corrective action takes place after a voltage error has arisen.